Cathode heaters of modern vacuum electron tubes generally include a length of wire formed of a refractory metal, such as tungsten, molybdenum, rhenium or a refractory alloy, such as tungsten-rhenium. The wire is usually bent into a convenient shape, such as a flat spiral or a zig-zag configuration, or a cylinder or toroid.
For proper operation, the heater wire is electrically insulated from the cathode which it heats, as well as from any supporting structure of the heater itself. Electric insulation between the heater and the remaining components is typically provided by maintaining an adequate distance between the heater and the remaining structure, as in the case of a free-standing heater. Alternatively, the heater wire is coated with an insulating layer, such as alumina, as in the case of a cataphoretically coated heater. In still another arrangement, the heater is electrically insulated from the surrounding structure by placing a separate insulation component between the heater and the surrounding structure, as in the case of a captured heater. A further structure for electrically insulating the heater from the surrounding structure involves embedding the heater wire in an insulating potting material, as in the case of a potted heater. Combinations of the above structures are widely used. In summary, the typical modern prior art heater for an indirectly heated cathode of a vacuum electron tube is basically an insulated, bent wire made of refractory, electrically conducting material that is electrically insulated from structures in proximity to it.
For most modern indirectly heated cathode applications, the bent electrically conducting, refractory heater is suitable. A disadvantage, in certain situations, with the typical modern heater is that it requires a substantial length of time, such as one minute, to achieve the temperature required for emission of electrons from the cathode. There are certain applications wherein the cathode must achieve emission in a matter of seconds or which require a greater efficiency in transferring heat to the cathode. In addition, there are certain applications in which the entire vacuum tube, including the heater, must be able to survive severe shock and vibration loading.
While potted heaters seem to be the best able to withstand shock and vibration loading, the potting material has a great deal of thermal mass. The potting material thermal mass substantially increases the tube warm-up time, i.e., the time between the initial application of current to the heater and the emission of electrons from the cathodes. While tube warm-up time can be reduced by decreasing the amount of potting material, this solution is not usually satisfactory because the reduction in the potting material weakens the integrity of the structure, thereby making it prone to failure due to thermal and mechanical shock. While the other types of heaters mentioned above have low thermal mass, they cannot, to our knowledge, be made to survive severe shock and vibration loading.
Pyrolytic graphite, which is manufactured by chemical vapor deposition at high temperatures, has been suggested and attempted for heaters of indirectly heated cathodes. It was thought that pyrolytic graphite heaters would enable the goals of fast warm-up and mechanical ruggedness to be attained because the resulting material is laminar and exhibits extreme anisotropic material properties. The structure of pyrolytic graphite is characterized by basal planes wherein carbon atoms are arranged in a precise hexagonal pattern. The basal planes of single-crystal graphite are orderly stacked, but the planes of pyrolytic graphite are somewhat randomly stacked.
The direction parallel to the basal planes, known as the "a" direction (which is the direction of the crystallographic "a" axis), is characterized by high tensile strength, low thermal expansion, high thermal conductivity and moderate electrical conductivity. For example, pyrolytic graphite deposited on an insulating substrate at a temperature of 2100.degree. C. exhibits the following characteristics in the "a" direction at 25.degree. C.
TABLE I ______________________________________ Value in "a" Property Direction ______________________________________ Electrical 700 .times. 10.sup.-6 Ohm-cm resistivity Thermal 3300 BTU/hr-ft.sup.2 -F/in conductivity Linear thermal 1.5 .times. 10.sup.-3 in/in expansion (from 0.degree. C. to 1000.degree. C.) Modulus of 4.29 .times. 10.sup.6 psi elasticity (pure tension) Tensile 10 .times. 10.sup.3 psi strength Compressive 15 .times. 10.sup.3 psi strength ______________________________________
In the past, heaters and other elements of discharge tubes have employed pyrolytic graphite wherein the planes of pyrolytic graphite are stacked or deposited in the "a" direction on a thin anisotropic pyrolytic boron nitride (APBN) substrate. In the latter instance, the graphite is selectively removed so that only a sinuous conductive pattern remains on the insulating surface of the APBN substrate. Electric current passes through the pyrolytic graphite in the "a" direction. While heaters of this type can be made very thin for fast warm-up, there are problems. Adhesion between the pyrolytic graphite and APBN substrate is very weak, whereby thermal stresses during warm-up often cause the pyrolytic graphite layers to separate from the APBN and themselves. To establish electrical connections to the heater, leads are brazed directly to the pyrolytic graphite surface. It has been found very difficult to achieve good brazes with pyrolytic graphite because of the smooth laminar structure thereof, as well as the low tensile strength of the pyrolytic graphite in the plane at right angles to the "a" direction; the plane at right angles to the plane of the "a" direction is known as the "c" direction, which is the direction of the crystallographic "c" axis.
Pyrolytic graphite, in isotropic form, was employed in the latter part of the nineteenth century as a filament in incandescent electric lamps. In the beginning of the twentieth century, tungsten replaced pyrolytic graphite as the preferred material, causing pyrolytic graphite to fall into disuse as a heater for electric lamp filaments. Since the beginning of the twentieth century, tungsten has become the standard material for heating filaments for most applications, including electron guns, i.e., cathodes, for microwave tubes. Tungsten replaced pyrolytic graphite because the graphite had a tendency to separate from a carrier or substrate therefor; further, the layers of anisotropic graphite separated from each other.